Balance, gait, speech, and swallowing deficits are major causes of disability in patients with Parkinson's disease (PD). In many patients, these "axial symptoms" are resistant to levodopa therapy. A potential advantage of deep brain stimulation (DBS) of globus pallidus internal (GPi) and the subthalamic nucleus (STN) could be improvement of these axial motor symptoms. Our long-term goal is to determine the effects off DBS on axial motor control in order to understand the role of the basal ganglia in centrally (voluntarily) and peripherally (automatically) initiated axial movements. We hypothesize that there are multiple motor outputs from the basal ganglia with differing sensitivity to dopamine and DBS. The goal of this project is to distinguish functionally different basal ganglia control mechanisms for the axial motor system by their unique patterns of response to DBS and levodopa when used separately and when used in combination. We hypothesize that DBS will he more effective than levodopa and that STN stimulation will be more effective than GPi stimulation for axial parkinsonian symptoms. This project will take advantage of a unique opportunity to rigorously quantify the effects of chronic DBS in radiologically-identified human basal ganglia in a randomized, double-blind study. These studies will quantify forces and movements in two types of postural and oromotor tasks: peripherally triggered, automatic responses to external cues and centrally-initiated voluntary movements. The specific aims are: (1) To determine how DBS affects centrally-initiated and peripherally-triggered postural control. (2) To determine how DBS affects centrally-initiated and peripherally-triggered oromotor control. (3) To investigate the interactions of DBS and levodopa treatment on postural and oromotor control. (4) To determine effects of the site (GPi versus STN) of DBS on postural and oromotor control. These experiments will quantify the efficacy of DBS and levodopa both separately and when combined for balance and oromotor deficits in patients with PD. Quantifying two types of postural and oromotor control in the same subjects will also substantially increase our understanding of the role of the basal ganglia in axial motor control.